1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory device and a manufacturing method for the same. More specifically, the present invention relates to a non-volatile semiconductor memory device in which data can be electrically rewritten, and a manufacturing method for the same.
2. Description of the Related Art
A flash memory that represents a non-volatile semiconductor memory device in which data can be electrically rewritten will be described below. A flash memory has: a first insulating film 2; a first polysilicon electrode 3 that becomes a floating gate; a second insulating layer 4 formed of a lamination made of an oxide film, a nitride film and another oxide film; and a second polysilicon electrode 5 that becomes a control gate which are laminated on a semiconductor substrate 1 in this order shown in FIG. 4.
It is noted that a source and a drain are formed in the surface layer of the silicon substrate 1 that correspond to the both ends of the floating gate although the source and the drain are not shown in FIG. 4. In addition, FIG. 5 shows an equivalent circuit diagram of the flash memory of FIG. 4.
This flash memory allows for rewriting of data through release and injection of electrons via the first insulating film 2 by utilizing the tunnel phenomenon; therefore, the first insulating film 2 is also referred to as a tunnel insulating film. The first insulating film 2 usually has a film thickness of approximately 8 to 20 nm.
The writing operation is carried out by applying a voltage of 4 to 6 V to the drain and by applying a voltage of 10 to 12 V to the control gate for 1 μsec to 5 msec. Electrons within the channel in the semiconductor substrate are accelerated by the electrical field between the drain and the source under the above described conditions so as to become hot electrons which jump over the forbidden band in the first insulating film 12 and are injected into the floating gate 5. As a result, the floating gate 5 is negatively charged and the threshold voltage of the control gate 10 becomes of a high value between 3.5V and 5.5V. The written program is carried out in such a manner.
On the other hand, the erasing operation is carried out by applying a voltage of −6 to −8V to the control gate, by applying a voltage of 4 to 6V to the source and by setting the drain in the floating condition. A tunneling phenomenon occurs under such conditions through the first insulating film 12 in the portion where the source and the floating gate 5 overlap with each other. Therefore, electrons within the floating gate are extracted to the source region, so that the erasing operation is carried out and the threshold voltage becomes of a low value between 1 V and 3 V.
In addition, Japanese Unexamined Patent Publication No. 2001-160555 discloses a non-volatile semiconductor memory device having the same structure as described above, wherein the first insulating film 12 is formed solely of a silicon nitride film made of radical nitriding species.
The writing speed according to the above-described hot electron injection system is determined by the gate current Ig of the electrons which are injected into the floating gate according to this system. The Ig is specified by the following expression according to an electron temperature model.Ig=C(Eox)·Id·exp(−qφb/kTe)  expression (1) 
Herein, C(Eox) is a function which is proportional to the injection probability and which depends on electrical field Eox applied to the gate insulating film between the drain and the gate. Id denotes a drain current, φb denotes the height of the barrier of the gate insulating film, k is the Boltzmann's constant, and Te is the electron temperature.
As described above, the writing speed is determined by the injection probability C(Eox) of the hot electrons, the drain current Id and the height φb of the barrier of the gate insulating film. In addition, the injection probability C(Eox) is substantially determined by the structure of a cell of the flash memory because the reliability of the gate insulating film is regulated by electrical field Eox at the time of the writing.
The drain current Id at the time of the writing is represented as follows.                     Id        =                                            W              L                        ·            μ                    ⁢                                           ⁢                      n            ·                          Cox              ⁡                              [                                                                            (                                              Vcg                        -                        Vth                                            )                                        ·                    Vd                                    -                                                            1                      2                                        ⁢                                          Vd                      2                                                                      ]                                                                        expression        ⁢                                   ⁢                  (          2          )                    
The drain current is also substantially determined by the structure of the cell as long as a thermally oxidized silicon film is used for the gate insulating film. That is, a channel length L in the W/L term is regulated by the limitation of miniaturization by the processing technology. The expansion of the cell size becomes necessary in order to increase a channel width W which is against the intention such as miniaturization and cost reduction generally when flash memory are manufactured. Mobility μn does not greatly change as long as a silicon oxide film is formed on the silicon substrate by means of thermal oxidization as the gate insulating film and, in addition, the height of the barrier of the gate insulating film is constant as long as the silicon oxide film is used.
Thus, it has been difficult to simultaneously implement miniaturization of cells and an increase in the writing speed in the case where a flash memory that is written according to a hot electron injection system is formed.
In addition, the tighter the bond of Si—N in the nitride film formed of radical nitriding species is, the higher the barrier property of the nitride film itself against nitriding species in the silicon nitride film formed of radical nitriding species described in Japanese Unexamined Patent Publication No. 2001-160555. Therefore, the diffusion of the nitriding species is blocked and the growth rate of this nitride film tends to be saturated, making it difficult to obtain an arbitrary thickness.